rotarygod

You read it right although I have never seen it written anywhere. I usually hang around the RX-7 club forum but thought that since I am a rotary fanatic, and everyone here should know everything about the new RX-8, that this would be a good place to post this.

I've done alot of study on how intakes and exhausts are designed and why they work the way they do. I know how to design the proper length intake runners for any engine as long as I know the port timing and desired peak rpm desired. I also understand the effects of Helmholtz resonant tuning and how to design it too. So here's what I discovered through my study of the Renesis induction system.

First, I read on here somewhere that Racing Beat has a new RX-8 and has been testing different types of air filters and intake combos. After some experimentation they have found that the stock system works best. Why? Helmholtz resonant tuning! The air filter housing isn't just big because the filter is huge. It is this size for a reason. Here's how to design a Helmholtz plenum chamber and some basic rules for it. When someone gets their new car, go out and measure both intake ducts for length and compare them to these numbers. If I'm right we should be very close. My numbers are generalizations and intended as ballpark figures. It doesn't change the principle.

Since we are dealing with 2 rotors we will just assume that the engine compares to a 2 cylinder. This is only a number for intake runners sake and not the total number of combustion faces per rotor.

As a general rule Helmholtz plenum volume should be sized as follows:

2 cylinder engine: Equal in size to total displacement of the engine regardless of stroke. For a 2 rotor use 240 cu. in. (3.9 liters). )Please don't start a debate on this!!! I have 2 articles that I will post here later to show how this was deduced.)

4 cylinder engine: Helmholtz plenum volume should be 50%- 60% of the total engine displacement.

6 cylinder engine: Helmholtz plenum volume should be 65%- 80% of the total engine displacement.

8 cylinder engine: Won't get into it now.

Back to the rotary. Diameter of the air intake tube should be sufficient in size that at the peak power rpm, air moving through the plenum intake tube (not the intake runners) should not exceed 122.73 mph.

Now for the plenum intake tube length. Just taking a short cut and getting to the good part: The tube would have to be around 7" long for a 10,000 rpm tuning. If we want to tune it for a lower rpm, we add about 1.7" in length to the tube for each 1000 rpm we go down. For every rpm higher we subtract about 1.7" in length.

Now go look at a picture (or the real thing if you have the car!) of the intake tubes leading into the air box. I am speaking for the 250 hp, 6 port car. There are 2 intake tubes. One really long one and one short one. The short one opens up at 7250 rpm for a shorter air intake route. Why would that matter? You know why now! If we measure the long tube length and compare it to my above numbers we can pretty much determine what rpm the designers intended it to benefit. The same thing can be said of the short tube.

The air filter will play a small affect on what the chamber resonates at. The filter would act a little like poly fil added to a speaker box. It would make the box act as if it were slightly larger than it really is. This is negligible though.

So here's the lowdown on how a Helmholtz plenum works. It doesn't need to be the intake manifold plenum. The only things that affect the tuning of the system are; plenum volume, air inlet length, and air inlet diameter. Notice that it is not affected in any way by port timing. The whole point is to create a resonance at a certain rpm. In the case of the new RX-8, 2 different rpm's. This resonance is the point where the air in the chamber is at its lowest pressure. Ah, now you understand! Lower pressure promotes greater airflow into the chamber. More airflow into the chamber is more airflow out of the chamber and into your engine! The same principle is what makes the VDI system work. You could add a Helmholtz plenum the top of a Holley carb or a Weber carb and it would have the same effect!

Now you all know! Buy a drop in K&N filter but leave the factory intake design alone! Just say no to cone filters for the RX-8!

P00Man

what do you mean by resonance? and whats a plenum?
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WEBSITE DESIGN

mazdabob

Thanks for the info! The automattic car has less power and a 7500 rpm redline, so I assume the intake tubes are different. Does this have anything to do with the AT car having less HP, but more torque? (164 vs. 159)

wakeech

no, the torque issue has to do with the porting.

great stuff rotarygod. i've always wondered about the manifold-forward kinda stuff... any books (yes, heavy stuff is A-OK) that you'd suggest for more on all-motor tuning??

QuantumTheory08

...resonance is typically an accoustical phenomenum that happens with air and sound at a specific frequency in an enclosed chamber (think pipe organs). Every pipe organ has a specific resonant frequency (how it is tuned for whatever tone (pitch or frequency) it was designed to produce; e.g. large piper organs produce low pitch (longer wavelengths) and tiny pipe organs produce high pitch (shorter wavelengths).

Another example of resonance is if you blow over a soda bottle half full - you can get it to "whistle". If you drink a bit of the soda, the pitch will be lower due to the chamber becoming larger.

BMW uses this principle on it's 7 series motor where the length of the chamber(the "plenum") is continuously changed for the matching RPM of the engine.

There is resonance for open ended tubes and close ended tubes. It is a great experiment to show people, how resonance works with an ordinary piece of PVC pipe, cut to the exact length to match the resonating frequency for a specific tuning fork. If you hit the tuning fork, you can barely hear it, but put it up by the open pipe of PVC and it sings like a pipe organ!

Resonance is almost like the air is perfectly tuned to the chamber and thus exits the tube perfectly, or with some added "umph!".

That's as much as I know. I have not heard of Helmholtz tuning before, but it sounds like the plenum lengths correlate to the valves on a trumpet (like the Renesis) where the BMW is more like a trombone (where you can slide the tube to increase or decrease the chamber length).

roachman

Man, you are making my head hurt on a Monday :o)

This is very cool stuff. I cannot wait to bring it up in a conversation....

All joking aside. Thank for the information.

And, does this mean the intake will "sing" at the right rpm?

Roachman

rxeightr

That is what the rotary is best known for, and the RENESIS will be no exception.

What beautiful music it will make.

Racer X-8

Mazda is no stranger to variable pitch intakes. I had the most fortunate opportunity to tour their IMSA shop in Charlotte before they bowed-out. Those engines had variable intake stacks. Beautiful!!! MAJOR BUCKS!
Resonance "packs" the intakes in a natural kind of way, so it doesn't rob HP in doing so un-naturally, like turbo's. Of course, turbo's definitely pack more for an even bigger bang, but if you don't have a turbo, then you better work on the resonance.

Most excellent info! Thanks!

rxeightr

Over the weekend I picked up the RX-8 Book sent to us a few weeks back -- it talks about how the air intake is set up, that once the intake port of the 1st rotor is closed off some percussion takes place, and it sends air back through the intake at the speed of sound, and actually helps charge with more air the 2nd rotor as it enters it's intake cycle.

They stopped short of calling it 'turbo'charging tho.

wakeech

...years later, the R26B used very variable length runner trumpets (yes, the trombones) to the same effect. very wicked awesome.

rotarygod

I have to edit what I posted. The info is correct but the numbers given assume that the Helmholtz plenum is the main intake plenum which it is not on the RX-8. However, the principle and effect are still the same none the less and the volume of the air cleaner assembly is directly proportional to the inlet tuning frequency of either length inlet pipe. Still does the same thing!

The lower horsepower number from the auto car is two fold. First, the engine is only a 4 port! This gives it less total intake runner diameter as well as less total port timing. Is best suited for a lower tuning. It should be stronger up to its redline cutoff than the same rpm of the 6 port engine. Go look at the dyno charts Mazda published. It is!!! Also since the resonance effect that is given by the shorter inlet is tuned somewhere in the 7500-8000 rpm range, there is no point to having it on the lower horsepower car since its redline isn't even high enough to benefit from it. Tune it where the car will be driven most of the time.

rotarygod

Wakeech: If you would like some good info on designing intakes and why they work the way they do, check out any books written by David Vizard. He has a couple usually sitting in the automotive section of most big book stores. They do require a bit of study though and can't be fully understood by just casually reading them. I have sat down and spent hours and hours of studying like I was being graded on it. I even have a couple of articles I have written if you would like me to post tem here. Even when you learn the formulas though you still need to know how to adjust them for the rotary engine. I know how! If you ever have any questions PM me and I'll be more than happy to show you how.


I forgot to fully mention the VDI thing and how it works. VDI is a brilliant system based on the speed of sound and acoustics that Mazda uses. The neat thing is that it is not affected in any way by port timing. It is only rpm based. If you want a boost from the effect at 7000 rpm it makes no difference what kind of porting style you have! It is only set at a certain distance to tune for a certain rpm and thats it. Typically you want the VDI effect set about 1000-1300 or so rpm below your tuned length intake runners peak horsepower point. Lets look at the Renesis. The high horsepower engine makes peak horsepower at 8500 rpm. this would mean that we would want the VDI effect to come into play at somewhere between 7200-7500 rpm. Where did Mazda open it? 7250! Hot damn the numbers fit! It is very easy to design an intake manifold that utilizes VDI. We only have to know what rpm we want it to work at and then calculate from there.

8_wannabe

How could this knowledge be used to win a bar bet?

rotarygod

Do all your research before you make the bet so you are 100% sure you are correct! Just don't tell the other party.

Racer X-8

Exactly! Right on... I practically begged them for a job! Told them I'd be packed-up & back within 24 hours. Maybe I should have played hard-to-get. Er...uh...

r0tor

iTrader: (1)

Mazda has been using resonance tuning for a long time now. Even my Probe (w/Mazda engine) has a variable length intake controled by Mazda's VRIS (Variable Resonance Induction System) which opens up secondaries when the intake resonance reaches a certain point.

QuantumTheory08

All I know is that total abstinence is the only sure way not to get VDI. One must be careful about these things. Good timing won't cut it either.

Racer X-8

Couldn't have expounded any more expediently if I do say so myself, Q.

By the way, which one of those fine gentlemen might you be, pray tell?

Oh yeah, well my Mazda with a Probe engine doesn't. (I think) Does it?

Checkout that snazzy race car previously eluded to via search engines like I did earlier. I don't tink you got dat.

QuantumTheory08

...I'm the cop taking the photo in front of my squad car. (Note the dual port ram air inductance system this guy brought with him).:p

P00Man

i know what resonance is, but had NO IDEA how it applied to engines.

so lemme see if i have this straight

when the frequency of the air matches the rpm they act in harmony without anything causing any disturbances so exit more forcefully?

edit:
that doesnt sound right, maybe i misread and RPM has nothing to do with it except for the frequency of the air vibrating so then it goes to a diff sized chamber or something?

edit:
ok, i have no idea how rpm ties in with this, i dont know enough about engines.
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couple Cam

P00Man

wow, i feel like a moron...
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Redhead live

Racer X-8

Hmmm. Think of a meatball on a rubberband. It boing-boing-boings up and down at a steady beat. That's the meatball/rubberband system resonant frequency.
A short rubberband or a smaller meatball = faster boing.
A long rubberband or a bigger meatball = slower boing, right?

Now, if your mouth was placed under the bouncing meatball, and everytime it reached the bottom of it's boing, you ate it (it's a magical meatball that doesn't go away when you eat it). If it's boinging too fast, you get bonked on the nose a lot, and that's not good. If it's boinging too slow, you go hungry, and that's not good either. If you boing it at just right frequency, you wind up getting fed more than just sitting there with a fork. Now, just change the meatball with intake air mass (blob of air), and your mouth with the intake ports. See?

OK, now, the intake air mass is kind of like water-hammer. You know, when you shut off a faucet, and when the flowing water reaches the new dead end, its kinetic energy, oops, I mean its weight gotta come to a stop fast. When it does, it "hammers" pipes & stuff. The speed that it does that (no, it's fast, but really not instantaneous) is totally dependant on the pipes & stuff that it's flowing in. If you practiced a lot, you could get good at opening the water faucet just at the moment the water's gonna hammer it. What you would get is a big splash coming out, fast (just like the comedy where the girl opens the door just when the guy's gonna smash it open. He he.)

You get it now? The intake ports open just when the air mass gets to it, and like the guy blasting through the open door, the air blasts through the intake ports. That's good.

Now, when the RPM's change, you'll want to change the frequency of the air/ports system, like the meatball/rubberband. If you ain't got the big gucks for variable pitch intake ports that get shorter for higher RPM's, then you gotta settle with one resonant frequency that's tuned to a strategically chosen RPM, like 7250.

Whew

P00Man

ahhh
so it has nothing to do with acoustiscs so much as mechanics

the opening/closing of the ports is in perfect harmony with the intake of air so that where ever that leads to (rotors in the housing?) is constantly supplied with the optimum amount of air

i really dont know that much about engines, but im starting to peice it together pretty good i think, so when i put a "?" after a statement, could you say if im correct or not?
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LovelyWendie99

SA22C

Mazda has used intake pulsations to, in effect, 'supercharge' the rotary engine since the 13B powered GSL-SE model in 1983. The 'Dynamic Intake Chamber' on the SE helped the car make 135 hp and 135 ft/lbs of torque. This intake system was then refined for the 2nd generation RX-7, yielding a final hp of 160 by the end of the 2nd gen run.

rxtreme

iTrader: (1)

Resonance tuning is used on exhaust manifolds (headers), as well. It's been said a good header is the best HP/dollar ratio you can get.